Specific error for unsized dyn Trait return type

Suggest `impl Trait` when possible, and `Box<dyn Trait>` otherwise.

src/librustc_error_codes/error_codes.rs

@@ -608,4 +608,5 @@ E0745: include_str!("./error_codes/E0745.md"),
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
     E0727, // `async` generators are not yet supported
     E0739, // invalid track_caller application/syntax
+    E0746, // `dyn Trait` return type
 }